1. Field of the Invention:
The present invention relates to an improvement in the structure of a slide type exhaust brake which is interposed midway in the exhaust pipe of a large-sized vehicle such as a truck, bus or the like to close the passage of exhaust gas flow when the vehicle is going down a steep slope to thereby apply the brakes to the vehicle.
2. Description of the Prior Art:
AS a prior art of the above-described slide type exhaust brake, the one disclosed in U.S. Pat. No. 4,205,704 is known. Referring to FIG. 2, which is a fragmentary sectional view of the prior art disclosed in the aforementioned U.S. patent, a housing 11 of the exhaust brake defines a flow passage 12 of exhaust gas and a guide passage 13 for receiving a plate-shaped valve member (main gate) 14 and guiding the reciprocating motion thereof, the guide passage 13 being communicated with the flow passage 12 so that the valve member 14 extends into and retracts from the flow passage 12 in a direction substantially perpendicular to the axis of the flow passage 12. The valve member 14 has an exhaust pressure regulating bore (waste gate port) 15 extending through the bottom of the gate surface so as to communicate with the section of the flow passage 11 at the downstream side of the valve member 14 in terms of the direction of flow of the exhaust gas. The valve member 14 further has a pair of abutting walls 14' and 14" (webs) projecting from its front, or upstream, surface. A piston rod 16 of an air cylinder which is directly secured to the upper end (as viewed in the figure) of the housing 11 is passed through the abutting wall 14' so that the piston rod 16 is movably engaged with the wall 14' and the other abutting wall 14" is pressed by the distal end of the piston rod 16. A block member (waste gate) 16' is provided on the piston rod 16 so as to be located in the space defined between the abutting walls 14' and 14", the guide wall 16' abutting against the wall 14' when the flow passage 12 is opened. The block member 16' has an auxiliary valve member (waste gate seal) 16" which is adapted to close the exhaust regulating bore 15 in response to the operation of the valve member 14 for closing the flow passage 12. When the flow passage 12 is to be opened, the auxiliary valve member 16" is moved, before the valve member 14 is moved upward, to open the exhaust pressure regulating bore 15, thereby relieving part of the accumulated exhaust gas to the downstream side of the valve member 14 and thus lowering the exhaust pressure acting on the valve member 14. Thereafter, the valve member 14 is activated to open the flow passage 12.
The above-described prior art suffers, however, from the following problems. Since the exhaust pressure regulating bore 15 is provided in the bottom of the gate surface of the valve member 14, the mechanical strength of the valve member 14 itself is lowered. Further, the repetitive opening and closing operations of the valve member 14 in use causes wear of slide contact surfaces. In particular, the area of slide contact between the front surface of the bottom wall of the valve member 14 and the bottom surface of the auxiliary valve member 16" suffers from considerable wear. When these slide contact surfaces become worn, the auxiliary valve member 16" is incapable of tightly sealing the exhaust pressure regulating bore 15 when the valve member 14 closes the flow passage 12, resulting in the braking function lowering early, disadvantageously.